Plants are known (see for example document GB 2 136 330) comprising:
an endless conveying line,
a plurality of pallets movable along the conveying line, each receiving a number of mechanical parts of the motor-vehicle along a first section of the conveying line,
means for loading a respective motor-vehicle body on a respective pallet at a loading station, located at the beginning of a second section of the conveying line, downstream of said first section with reference to the direction of movement of the pallets,
at least one bolting station arranged in said second section of the conveying line and provided with means for bolting said mechanical parts to said body, and
means for unloading the respective body with the mechanical parts bolted thereon at the end of said second section of the conveying line.
In the above mentioned solution known from the aforementioned prior art document, the first and the second section of the line are part of an endless path arranged in a horizontal plane and they extend according to two directions parallel and spaced with respect to each other. At the ends thereof, the two parallel sections of the line are connected to each other by conveying lines directed transversely with respect thereto. The plants of this type have the drawback of occupying a relatively extensive space within the assembly plant. A further drawback lies in the fact that the abovementioned connection transverse sections, though occupying useful space in the plant, are entirely passive, in the sense that no operation useful within the assembly cycle is carried out there along. A further serious drawback lies in the fact that access to the useful sections of the line from the side facing inwards the endless path is difficult, hence the logistic management of the plant is complicated, especially regarding supplying parts to the various stations of the line.